The present invention relates to solid state image sensors. In particular, the present invention relates to prevention of image artifacts in solid state X-ray detectors such as those used in medical diagnostic equipment.
Present day solid state X-ray detectors are composed of an array of rows and columns of individual sensing elements (“pixels”) typically formed from amorphous silicon diodes. Each pixel is connected to a sense line (shared by a column of pixels) through a switch typically implemented as a thin film transistor or diode. Each row of pixels (a scan line) shares a separate control line, which activates and deactivates the switches for all the pixels in the scan line.
Readout electronics read the array by enabling one scan line at a time using the scan line for that row. Sensing electronics connected to each sense line measure the signal on each of the sense lines to provide a value for each pixel in the scan line. After the readout electronics read a scan line, that scan line is deactivated and the next scan line is read. The readout electronics read each scan line in succession until all of the pixels of the array are read.
When a switch is activated, electric charge from both the pixel and the sensing electronics is drawn into the switch to establish a conductive channel in the switch. The charge is ideally completely pushed back out of the channel when the switch is deactivated. However, because the switch is made from amorphous silicon, a relatively large amount of charge remains in the channel (“the retained charge”) and bleeds out slowly over time.
The retained charge affects the offset of a detector that has not been exposed to X-rays. An image that is read out from the array without first exposure to X-rays is called a “dark image”. The dark image, because of the retained charge, is slightly negative (i.e., there is a negative offset). Furthermore, assuming that there is time allowed for exposure between two readouts of the array, the first scan line read will have considerably less retained charge adding to its signal than the last line read (which will have retained charge from potentially all of the scan lines).
The sense electronics must accumulate and measure small amounts of charge transferred from the pixel during the time that the switches are activated. Furthermore, in order to avoid interference with the signals from one scan line to the next, the charge accumulated and measured by the sense electronics must be reset to zero after deactivation of one scan line and before activation of the next scan line. An integrator (formed by an operational-amplifier and a capacitor in the feedback path of the operational-amplifier) is often used to implement the sensing electronics. A switch across the capacitor may then be used to short out the capacitor and return the accumulated charge to zero. The output voltage of the operational-amplifier is a measure of the amount of charge integrated during a predetermined integration period when the switch is open.
One of the two terminals of the pixel (e.g., the diode cathode) is connected through its switch and the data line to one of the inputs of the operational-amplifier. The anode of all the diodes are held at another common potential. The intent is to form a bias across the diode, that being the difference in potential between the common and the data line (amplifier input). If the data line potential varies from one reading to the next, then the sense electronics will generate a signal that does not correlate to the X-rays absorbed by the detector (i.e., an error signal). The operational-amplifier works to keep the data line at the same potential as its second or reference input by supplying charge from the operational-amplifier output through the capacitor. However, the operational-amplifier is limited by its power supply voltages. Thus, if the operational-amplifier saturates, and can no longer supply charge to the feed back capacitor, the data line potential will change, thereby changing the bias across the pixel. An error signal results.
While the operational-amplifier is saturated, the operational-amplifier temporarily loses control over its input and the data line potential. In other words, the operational-amplifier cannot immediately restore the desired potential to the data line. As a result, many subsequent pixel readouts in a column are corrupted by the error signal, resulting in “white streaks” in the image read from the array. The streaks persist until the data line is returned to the desired potential. In the past, operational-amplifier saturation was commonly caused by allowing the integrator to accumulate charge on the capacitor for the relatively lengthy period between frames.
A need has long existed for a method and apparatus for preventing image artifacts that addresses the problems noted above and others previously experienced.